The problem with degradation and breakage of the carbon black pellets and the build-up of carbon black dust and fines on surfaces of the handling apparatus has existed since the beginning of automated feeding and weighing of carbon black for the injection into a mixer.. This problem has become even more critical in recent years due to the more sophisticated and unique rubber compounds. These newer compounds require softer, more fragile carbon black pellets which, due to their softer nature, will disperse more thoroughly into the rubber batch. As the sophistication of rubber compounding technology has increased, complete dispersion of the carbon black is more critical than ever before. The softer the pellets, the more complete the dispersion within a given time frame during the mixing process.
In rubber mixing, various ingredients are injected into a mixer to be compounded into the rubber. By its nature, carbon black will not mix well in its original form. As produced, carbon black is a very fine (micron size) powder. If injected into the mixer in that form, it will simply float on top of the rubber and will not mix into the rubber batch. In order to overcome this problem, the carbon black, at the point of manufacture, is made into pellets. In the pellet form, the carbon black will then mix well into the rubber. First, the pellets themselves will mix, then the pellets will break down into powder and the powder will complete the total dispersion of the carbon black in the rubber batch. The making of the pellet is in itself a science. These small pellets are made according to very close specifications. By their hardness, mass strength, elasticity, as well as other technical considerations, mixing performance can be determined.
These pellets are extremely fragile and easily broken. When broken, the pellets become powder (normally called "fines"). As in their original form, these powders will not disperse into the batch, but rather will "float" on top of the rubber. These pockets of powders become major flaws in the final product which most often causes the total product to be scrapped. Secondly, when high concentrations of fines are allowed to enter the mixer the time involved to mix the batch can become indeterminate, thereby extending the manufacturing time to unacceptable levels. Further, the mixing requires very specific time and temperature control, otherwise the rubber will "cure" inside the mixer due to the higher temperatures reached during an extended mixing time. It therefore becomes highly significant to the success of the mixing operation that the pellets be handled in the most gentle manner possible, that any dust (fines) not be allowed to accumulate within the handling and feeding equipment where it may break away and be fed into the mixer.
When the pellets are broken, the resultant powders (fines) are highly prone to adhering to any surface with which they come into contact. This occurs primarily at any point within the system where material must be held for further process. The carbon black is metered closely by weight in conformance with a precise recipe, dependent upon the type of rubber compound being mixed. In the typical operation, a batch weight ranging from a few pounds to 500 or more may be required, with a tolerance of plus or minus 1% of total batch weight. The bulk densities of the carbon black pellets may vary from 20 to 45 pounds per cubic foot. These weights, once conveyed into the weight hopper, with variables both in bulk density and total amount must be fed within a very specific cycle time, usually less than 90 seconds. Should this time cycle become unpredictable, all downstream operations are jeopardized. The mixing process is a closely timed, continuous operation, each step dependent upon the timely completion of all preceding steps in the operation.
The "fines", if allowed to accumulate within the feeders create significant problems. First, the build-up within the feeders will break away from surfaces, be fed to the scale and injected into the mixer where it will not mix thoroughly, creating very high reject levels in the final product. Secondly, the build-up chokes off the feeder, thereby reducing the ability of the feeder to deliver accurate amounts within the required time. Further, extreme levels of build-up on feeder surfaces may create excessive maintenance shut-down time for cleaning and servicing the feeder.
As far as is known, there have been many attempts to use various types of flite configurations on screw-type feeders. Some types of known screw flites are as follows: The tapered pitch may be utilized for an even withdrawal from the bin. The profile of the screw in this situation is tapered toward the feed end. Another type is the graduated pitch section whereby the space between the flites gradually increases across the feed section so that material will enter the feeder evenly. There have also been attempts made to use rotary feeders. This is a vane type feeder whereby the rotor is essentially a star type design which accepts materials within the vanes and rotates to dump the load directly below. There have also been numerous attempts to use other types of feeders. In attempts to prevent the black being abraded against surfaces within screw feeders, various belt conveyor configurations have been tried. There have also been numerous types of pneumatic feeders which attempt to fluidize the materials and thereby not require any direct contact with any hard surfaces. Further attempts to prohibit build-up and pellet breakage have included vibrators, inclines, stainless steel and polished surfaces, synthetic coatings and laminations. In addition to these mechanical attempts, there have been several methods of applying different electrical charges to various parts of the equipment in order to control the ionic attraction of the material. These attempts have often added to the problem rather than contributing to the solution.
In reviewing the pellet making process, I learned that in the manufacturing process of making carbon black and pelletizing it, there are generally fewer problems with the material sticking to the handling equipment. A possible reason for this is because during the pelletizing process, the pellets are dried by hot air driers which substantially elevate the temperature of the pellets to drive off moisture used in pelletizing. The hot carbon black during the manufacturing process generally does not stick to the handling equipment. The purpose of the heating was not to avoid sticking but to evaporate the water from the pellets.
This invention solves the problem of the carbon black pellets sticking to the handling equipment during the rubber manufacturing process by the use of sufficient heat to repel the micron size particles. The heat is believed to accomplish the task due to several events. It apparently significantly reduces the kinetic friction between the flite surfaces and the carbon black particles being conveyed. The reduced kinetic friction reduces greatly the abrading of the pellets and minimizes the opportunity for ionic attraction and minimizes charge transfer during movement of the feeder screw. Also carbon black dust is hygroscopic. The heated surface upon which it is conveyed minimizes the opportunity for the particles to adhere due to surface moisture within the dust particles.
The entire screw assembly is electroless nickel plated in order to provide a surface least likely to provide the initial opportunity for the adhesion of dust to the flite due to higher kinetic friction. Prior methods of trying to solve the problem does not understand nor contemplate the use of a predetermined amount of heat nor do they use the other features of this invention. There have been a number of screw feeders and conveyor configurations in the past used for different purposes which may include a heated surface. However, with these devices, as far as is known, the purpose of the applying of heat is to heat the material being conveyed or for a specific treatment purpose. An object of the present invention is to prohibit the adhering of carbon black to the feeder surfaces of a screw type or other type conveyor by just using sufficient heat.
Another object of this invention is to minimize the opportunity for creating fines within the feeder. The design of the unit provides for a true perimeter along the entire length of the screw. The screw is centerless ground at a specific diameter about the centerline over its entire length, thereby prohibiting the flite edges from grinding the fragile pellets between the flite edges and the housing due to any eccentric movement of the flites.
The speed of the feeder is closely controlled. It is believed that carbon black pellets will begin their greatest degradation and build-up at the point the speed of the conveying flite exceeds 180 ft/min. peripheral speed of the screw. By maintaining a lowered speed, together with the plated and heated flite surface, low friction and minimum degradation is provided.
By inhibiting the build-up on the flite surfaces, the feeder is able to maintain its design feed rate so that a consistent and predictable mixing cycle time is provided. Another object is to improve the quality of the mixes and inhibiting of fines concentrations entering the weigh scale which in other feeders come from build-up breaking away from internal feeder surfaces. Another object is to minimize maintenance demands in service shut-downs due to inability to repel carbon black from caking on the surface of the feeder.
Other objects of the invention will be apparent from the following detailed disclosure.